You can’t replicate all of the admin level account functions, such as adding a new device or changing the geopresence boundary: those are reserved to the official app.
There are already a number of existing “dashboard” projects, including two very polished browser based third-party apps which are very popular. See the following recent discussion on those. (The topic title is a clickable link.)
That said, smartthings is currently in the middle of a major transition: new app, new backend platform, And the new stuff is still very much in the “work in progress” stage. There isn’t even complete documentation on it. 
But you can look and see what there is.
And here is the announcement on the coming changes:
Updates to the SmartThings Platform
So if you want to build yet another dashboard project based on the old platform, you are certainly welcome to do so. Just be aware that you may have to redo all of the work in 2021 once the old platform is retired.